Well-defined, periodic nanostructures have received considerable attention since they can serve as useful templates and scaffolds for nanodots, nanowires, magnetic storage media, semiconductors, optical devices, polarizers, and photonic materials. For this purpose, bottom-up approaches have extensively been studied because they can offer an efficient, cost-effective strategy to overcome the technological and economic limits associated with large-scale top-down approaches. The self-assembly of block copolymers (BCPs), one of the most promising candidates for this purpose, has widely been studied as the sizes, spacings, and morphologies of the nanostructures from the self-assembled BCPs can be effectively tuned by varying molecular weight and composition ratio of BCPs. As importantly, the versatilities in the properties of the blocks can be easily introduced by many well-known chemical techniques.
Block copolymers consist of chemically distinct polymer blocks which are covalently bound together along a polymer backbone. The block molecular connectivity of certain classes of block copolymers allows them to undergo efficient self-assembly to form periodic nanostructures having consistent domain spacing. Further, the composition and size of polymer blocks can be selected to control the physical dimensions, arrangement and chemical properties of nanostructure domains. Block copolymers may incorporate a range of block configurations, including AB diblock, ABC triblock, ABA triblock or larger combinations such as ABCBA pentablock, providing further versatility in achieving useful self-assembled structures.
For some practical applications in optical and electronic materials, for example, the alternating domain spacing of the self-assembled BCPs beneficially extends up to a few hundred nanometers. Thomas and coworkers utilized the partially cross-linked, conventional BCPs to prepare photonic band gap materials for visible wavelengths, but this normally requires the molecular weight (MW) of BCPs to be extremely large for the applications mentioned above. It is noted that, according to the model system for polymers with the MW over the critical entanglement MW, the viscosity of polymers gets higher abruptly as the MW gets larger due to polymer chain-entanglement, which yields a significant kinetic barrier for the effective self-assembly of conventional BCPs with high MW. For this reason, the defects might not be able to be effectively annihilated even upon longer annealing time due to the entanglement, and there could be degradation of polymer chains upon thermal treatment due to significantly increased annealing temperature and time to overcome the kinetic barrier.
Brush polymers (also called comb or graft polymers) are defined as grafted polymers with both relatively high MW and significantly dense and regularly spaced side brush chains attached to the backbone. Due to the significant steric hindrance between densely grafted side brush chains, brush polymers can have an extended backbone and exhibit a reduced degree of chain-entanglement compared to conventional polymers. Therefore, it is often favorable for brush polymers to self-assemble into well aligned and ordered nanostructures even though the MW of brush polymers is relatively high. There are three general methods to make brush polymers. In the “grafting from” approach, a macro-initiator backbone is first synthesized but there are limitations in the efficiency of its initiation and conversion of monomers. The “grafting onto” method, where the side chains and the backbone are separately synthesized and then coupled together, have difficulties in obtaining complete grafting due to increasing steric hindrance and the subsequent purification of unreacted brush side chains can be problematic. In the “grafting through” method, which is also called the “macromonomer (MM) approach” the side chains are synthesized with a polymerizable end group which is subsequently polymerized. This approach has many advantages over those ‘graft from’ or ‘graft onto’ approaches, but still contains drawbacks like not being able to obtain high MW and/or narrow polydispersity index (PDI). Recently, Grubbs and coworkers successfully reported a novel ring-opening metathesis polymerization (ROMP) exploiting the high ring strain of norbornene monomer and the high activity of Ru-based olefin metathesis catalyst to synthesize brush polymers with ultra-high MW, narrow PDI, and well-defined, structural architectures. [see, Y. Xia, B. D. Olsen, J. A. Kornfield, R. H. Grubbs, J. Am. Chem. Soc. 2009, 131, 18525].
It will be apparent from the foregoing description that block copolymer materials exhibiting useful physical, chemical and optical properties are useful for a range of applications including photonics, optoelectronics, and molecular templates and scaffolding. Specifically versatile block copolymer materials are needed that are capable of efficient self-assembly to generate structures supporting a range of target applications.